This disclosure is related to the field of neutron well logging measurements for determining petrophysical properties of subsurface formations traversed by a wellbore. More specifically, the disclosure relates to using spectral analysis of gamma rays induced by neutrons to determine one or more petrophysical parameters of such formations.
Neutron induced gamma ray spectroscopy has been used to determine mineral and fluid composition of earthen formations traversed by wellbores, among other uses. In some embodiments, a gamma ray detector may comprise a scintillation crystal made from materials such as thallium-doped sodium iodide, bismuth germinate, gadolinium oxyorthosilicate, among other materials. The scintillation crystal is optically coupled to a photomultiplier tube which generates a voltage pulse in response to a scintillation (flash of light) emitted by the scintillation crystal in response to detection of a gamma ray. The amplitude of the voltage pulse is generally related to the energy of the detected gamma ray. Output of the photomultiplier may be electrically coupled to a multichannel analyzer, which counts numbers of voltage pulses occurring within selected amplitude ranges, and thus numbers of gamma rays detected corresponding to energy level ranges. The numbers of gamma rays detected at various energy levels where such gamma rays are induced by neutrons imparted into the formation may be analyzed to evaluate the composition of the formations.
U.S. Pat. No. 4,394,574 issued to Grau et al. discloses principles of neutron-induced gamma ray spectroscopy techniques. The '574 patent provides a detailed flow chart of full spectral analysis, in which a filter technique is one of the steps. The filter technique disclosed in the '574 patent is used to account for the detector resolution degradation in a measured gamma ray spectrum in order to match standard gamma ray spectra. Filtering may be performed by convolution of the standard spectra with a Gaussian function. The standard deviation of the Gaussian function may be a function of gamma ray energy (i.e. spectrum channel). In this way, the resolution of the standard spectra is adjusted to better match the measured spectrum with a degraded resolution.
Many factors may cause the energy resolution of a measured spectrum to be worse than the resolution of standard spectra, such as a detector with worse intrinsic energy resolution and high operating temperature, among other factors. Energy resolution is not the only factor that may vary from one detector to another, or vary with respect to temperature. Recently, it has been discovered that the shape of a full energy peak (i.e., a localized maximum amplitude at a particular energy level) can deviate from a symmetric Gaussian shape to a skewed, non-Gaussian shape. Skew in energy peaks in a measured gamma ray energy spectrum can be caused by scintillation crystal non-uniformity including non-uniform light collection and scintillator dopant gradients, etc. The skewed, non-Gaussian shape of one or more energy peaks can also vary with respect to operating temperature.
Non-Gaussian energy distribution about one or more energy peaks has not previously presented a problem in spectral analysis of gamma rays because the intrinsic detector energy resolution of older types of scintillation detectors was insufficient to determine the exact shape of a full energy peak. Therefore, it was sufficient using older types of scintillation detectors to assume that any peak in the measured spectrum had a Gaussian shape. However, with newly-developed detector and signal acquisition technology, (see, Knoll, G. F., Radiation Detection and Measurement, John Wiley and Sons, Inc., Hoboken, N.J., 2010) the intrinsic detector energy resolution has been improved dramatically. The assumption of a Gaussian shape of measured energy peaks has been determined no longer to be accurate. Variations of the shape of measured energy peaks need to be taken into account for accurate spectral analysis. Failure to take into account the spectral differences caused by variations in skew of the energy distribution may lead to biases in the elemental yields extracted from the measured spectra and as a consequence biases in the computed elemental concentrations.